Let's say you have a flash drive with a file you want to transfer to your Documents file in Windows 7. You open up Documents; grab it at the top and drag it to either the right or left of your screen' once your cursor touches the edge of the screen, the window will "snap" to exactly 1/2 of the screen. Then you go to Computer and open up your flash drive; grab it at the top and drag it to the opposite side of your screen; once your cursor touches the edge of the screen, it will "snap" to exactly 1/2 of the screen just like the other window. Now you have two windows open, with each one occupying exactly 1/2 half of your screen. Now you can see exactly what is in each folder and can drag and drop between them or whatever else you want to do.

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In my PC corner for the winter, gaming, knitting and reading.

It's definitely not useful for having multiple windows open, but if you need to compare two documents or would like to work in two folders - such as dragging and dropping - it is probably one of the features I use most often in Windows 7 and Windows 8. This is another video from Microsoft that shows how to use it...